The present invention relates to a pulse shaping means. In particular the pulse shaping means is suitable for shaping the mater pulse associated with an optical backplane device.
The invention has application in the development of new architectures for optical switching applied to high-speed digital communication routers/switches.
UK Patent Application No. 9930163.2—“Data Compression Apparatus and Method Therefor” describes the operation of an optical backplane, for example an optical fiber, in an optical switching system. In particular, 9930163.2 discloses a compression method for converting packets of data at 10 Gbits/s to compressed packets at 1.28 Tbits/s. The compressed packets are then time multiplexed onto the fiber optic backplane of a device such as an IP router or ATM switch.
An essential part of a compression method is the generation of a chirped master laser pulse. This is typically a 5 nanosecond (ns) pulse that chirps over 5 nanometers (nm) wavelength. The ideal pulse shape, the pulse having substantially constant power, is illustrated in FIG. 1, using the parameters above.
The chirped master laser pulse can be generated using a laser that directly produces a chirp. However, the currently preferred method is to use a laser that has a very narrow pulse and to convert this to a chirp by propagating it through a dispersive transmission medium, for example a length of optical fiber or a Bragg fiber grating.
FIG. 2 shows the pulse profile for an ultra-short laser pulse approximately 100 femtoseconds (fs) in length. The spectrum of this pulse is shown in FIG. 3. The pulse can then be passed through a dispersive medium to generate a chirped pulse as illustrated in FIG. 4. The chirped pulse can be passed through an optical filter to block wavelengths at start and end of the pulse to give a pulse that chirps over the required 5 nm of wavelength.
The resulting truncated pulse is illustrated in FIG. 5, which may be compared to the ideal pulse in FIG. 1. In blocking wavelengths higher and lower than the 5 nm bandwidth the optical filter also attenuates the pulse at times corresponding to these wavelengths 510. It can be seen that the truncated, chirped pulse 500 does not have a constant power level.
The difference in power between the peak 502 and the edges 504 of the pulse in FIG. 5 can be reduced if a narrower ultra-short pulse is used, thus widening the spectrum of the pulse. However, this approach has the disadvantage that the amount of original laser power that is lost, in the truncation process increases as the ultra-short pulse is shortened.